Vitamin A (retinol) is essential for the maintenance of retinal photoreceptor cell structure and function. In the absence of this vitamin, photoreceptor cells degenerate. The rate of degeneration is fastest in the inferior hemisphere of the eye. Although light accelerates the rate of cell loss, the regional difference in rate of degeneration occurs independent of environmental lighting. Perhaps regional differences in rate of cell loss is related to a diversity in metabolism and/or stores of retinol or the interrelationship between photoreceptor cells and pigment epithelium.